1. Field of the Invention
The present invention relates to a receiver for use in a transmission system in which an analog signal and a digital signal are simultaneously transmitted at the same center frequency. More particularly, it relates to a receiver that selectively receives either an analog signal or a digital signal.
2. Description of Related Art
In general, a receiver that selectively receives either an analog signal or a digital signal in a transmission system in which the analog signal and the digital signal are simultaneously transmitted at the same center frequency is known. For example, there has been proposed a broadcast system called an IBOC (In Band On Channel) system by U.S. iBiquity, as a broadcast system in which an analog broadcasting wave and a digital broadcasting wave are simultaneously broadcast by an identical broadcasting station.
In this IBOC system, a digital broadcasting wave is transmitted via a channel between analog broadcasting channels, and a multi-carrier transmission method is used for the digital broadcasting of the IBOC system. In this case, in order not to affect the analog broadcasting channels which are adjacent to the digital broadcasting channel, the digital broadcasting wave is transmitted with low transmission power compared with that for the analog broadcasting waves.
On the other hand, a related art receiver that can selectively receive either an analog broadcasting wave or a digital broadcasting wave performs reception switching in order to receive the analog broadcasting wave when the receive status of the digital broadcasting wave gets worse due to fading, interference waves, or the like and the receiver therefore enters a nonreceipt status, and performs switching to reception of the digital broadcasting wave when the receiver enters a status in which it can receive the digital broadcasting wave while receiving the analog broadcasting wave.
By the way, since the transmission power of the analog broadcasting is larger than that of the digital broadcasting, as mentioned above, an analog signal having a large amplitude and a digital signal having a small amplitude coexist in a predetermined frequency range of an intermediate frequency signal (IF signal) generated from a transmission wave from the broadcasting station, and therefore the above-mentioned receiver that can selectively receive either an analog broadcasting wave or a digital broadcasting wave needs to separate the analog signal and the digital signal from the IF signal.
To this end, the related art receiver extracts a signal which falls within a frequency range close to the center frequency of the IF signal and which is associated with the analog broadcasting by using one of a pair of band pass filters, and extracts only a signal associated with the digital broadcasting by removing the signal which falls within a frequency range close to the center frequency of the IF signal from the IF signal by using the other band pass filter. The related art receiver then outputs either of these signals selectively.
Furthermore, there has been provided a related art receiver that attenuates only signal components associated with analog broadcasting from an input transmission wave by using a band eliminate filter (BEF), furnishes the transmission wave to an analog-to-digital converter (ADC), and carries out digital demodulation of a signal associated with analog broadcasting and/or a signal associated with digital broadcasting outputted from the ADC by using a digital demodulator. In this related art receiver, an AGC (automatic gain control) voltage generating unit controls the gain of an IF amplifier placed in the front of the BEF according to the output of the BEF so as to increase the gain of the IF amplifier, and increases the signal level of the digital broadcasting relatively (see patent reference 1, for example).
[Patent reference 1] JP, 2002-359569,A (see pp. 4 to 5 and FIGS. 1 to 6)
A problem with related art receivers constructed as mentioned above is that while they can easily receive digital broadcasting waves since they attenuate only a signal component associated with analog broadcasting by using a BEF, in a case where digital broadcasting waves are located in both sides (i.e., a high-frequency side and a low-frequency side) of an analog broadcasting wave, they may determine that they can receive the digital broadcasting waves even when a multi-pass or the like occurs in one of the digital broadcasting waves and the status of reception of the digital broadcasting wave gets worse since they determine the receive level of the digital broadcasting by measuring the levels of the digital broadcasting waves in the both sides of the analog broadcasting wave.
Another problem with related art receivers is that in a case of reception of analog broadcasting, since related art receivers attenuate only a signal component associated with the analog broadcasting by using a BEF, as mentioned above, the level of the signal component associated with the analog broadcasting decreases and therefore the signal-to-noise (S/N) ratio decreases.